42 Self and the Phenomenon of Life
b2726 Self and the Phenomenon of Life: A Biologist Examines Life from Molecules to Humanity “9x6”does not work at all in the laboratory.^30 A number of experiments
have been designed to bypass the obstacle. For example, Suther-
land’s group employed a highly tedious and complicated procedure
to obtain cytosine and uracil nucleotides, proving in principle that
activated ribonucleotides can be obtained without directly reacting
a nucleobase and ribose.^31 Nevertheless, many of the critical steps
require alternating exposures to mutually incompatible conditions
such as heating/cooling and dehydration/rehydration; these highly
precise and sequential steps were unlikely to occur in nature.
(4) The phosphodiester bond forming the backbone of the RNA mole-
cule is unstable in water and easily hydrolyzed. This is the so-called
“water paradox,” a situation referring to the fact that biochemical
reactions need to take place in an aqueous milieu, yet water is toxic
to the biopolymers. In modern life, damage done by water to RNA
is enzymatically repaired, but this mechanism was lacking in the
primordial world where protein enzymes are absent. It was found
that the solvent formamide (present in interstellar dust) stabilizes
nucleic acids. However, it is doubtful that formamide could be pres-
ent in such abundance on early Earth as to be able to support an
RNA world.^32
To sum up, despite the elegance of the dual-functionality of
ribozymes, the major drawback of placing an RNA world ahead of others
lies in the difficulty of explaining the origin of RNA itself.^33 To overcome
the impasse, some investigators proposed the presence of “pre-RNA”
replicative molecules that gave rise to the ribozymes, in effect negat-
ing the primacy of RNA in the origin of life. Following are some of the
pre-RNA hypotheses: (a) Cairn-Smith proposed that the first replicating
system was inorganic. Clays form naturally from silicates in solution, and
clay crystals preserve their external formal arrangement as they grow.
The irregularities of the distribution of cations (positively charged ions)
could serve as the repository of information. Replication would be pos-
sible if any arrangement of the cations in a layer of clay dictates the